1. Field of the Invention
The present invention relates to a test member for measuring the concentration of an analyte in a fluid sample, notably to a test strip for analysing blood glucose or other analytes in bodily fluids.
2. Background of the Invention
Diabetics regularly need to test samples of their blood to determine the level of blood glucose. The results of such tests may be used to determine levels of medication needed to treat the diabetes at the time. In one known type of system, disposable sensors are used to test the blood. The sensors typically take the form of test strips which are provided with a reagent material that will react with blood glucose to produce an electrical signal. Conductive tracks on the test strip relay the electrical signal to a meter which displays the result. After a sample of blood has been applied to the test strip and the measurement has been taken, the test strip is disposed of. In order to couple the conductive tracks on a test strip with the meter, the test strip needs to be inserted into a sensor holder prior to the start of testing. The sensor holder has corresponding electrodes which are brought into electrical contact with the conductive tracks of the test strip. Alternatively, the reagent in the test strip may undergo a visible color change, the magnitude of which is used to determine the analyte concentration in the applied fluid.
It is known to provide a stack of disposable circular test elements in a cylindrical housing, the stack being urged towards a test station by a spring to form a liquid-proof seal, for example as described in WO 94/10558.
A problem with providing disposable test members in a stack is that the working area to which the fluid sample will be applied can become scuffed, particularly when a compressive force is applied to the stack by a spring.
It is an object of the present invention to provide an improved test member suitable for use in test devices that employ test members in a stack.
According to an aspect of the present invention there is provided a test member suitable for use in a test device for testing of analyte concentration in a fluid to be applied thereto, the test member comprising a base member having a working area to which the fluid is to be applied, containing a reagent which is reactive to the said analyte to produce an electrical signal or a color change, and a non-working area adjacent to the working area, wherein the total thickness of the test member in at least a portion of the non-working area is at least as great as the total thickness of the test member in the working area.
By making the non-working area at least as thick as the working area, scuffing or abrasion of the working area in a stack can be reduced. Moreover, if a compressive load is applied to a stack of the test members, this may be spread out over a greater area, thereby reducing the possibility of compressive damage to the working area.
In a preferred embodiment, at least a part of the non-working area is of greater total thickness than the thickness of the working area. This further reduces the likelihood of damage to the working area by scuffing or abrasion when in a stack. The difference in thickness is preferably from 1 to 20 xcexcm, notably from 5 to 10 xcexcm.
The test member may be of any desired shape for a particular application; however, typically the test member will be an elongate test strip. For convenience hereinafter, the invention will be described with reference to such a test strip. However, it is to be understood that the invention is not limited to this embodiment.
In one embodiment, the reagent is reactive to the analyte to produce a visible color change. Alternatively, the reagent may react with the analyte to produce an electrical signal which is measured and displayed by a meter. In this embodiment, the working area has electrodes which are electrically connected to electrode tracks in the non-working area, and at least part of the tracks are exposed for connection to electrodes of a meter. The invention will be described hereinafter with reference to this embodiment.
To build up the working area, a plurality of layers are sequentially applied to the base layer, for example by screen printing, typically with curing or drying steps between the application steps. The layers which are printed typically comprise electrode patterns, a reagent layer, and a mesh layer (for spreading out an applied fluid). As a result of the application of these layers, the working area of a conventional electrochemical test strip is typically about 100 xcexcm thicker than the non-working area, which contains the electrode tracks and, typically, a dielectric layer. A stack of 100 test strips will therefore be about 10 mm thicker in the working area than in the non-working area. In a test strip in accordance with the present invention, at least a part of the non-working area may be made thicker by any suitable means. Suitable means include, for example: a printed relief ink; an applied pad or tape; embossing of the base layer or an intermediate layer; or an extension of the mesh layer from the working area.